linux-kernel - Re: [PATCH] include/log2.h: Fix rounddown_pow_of

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Date:	Tue, 15 Nov 2011 00:27:00 +0100 (CET)
From:	Jesper Juhl <jj@...osbits.net>
To:	Andrei Warkentin <awarkentin@...are.com>
cc:	linux-kernel@...r.kernel.org, Andrei Warkentin <andreiw@...are.com>
Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

On Mon, 14 Nov 2011, Andrei Warkentin wrote:

> ----- Original Message -----
> > From: "Jesper Juhl" <jj@...osbits.net>
> > To: "Andrei Warkentin" <andreiw@...are.com>
> > Cc: linux-kernel@...r.kernel.org
> > Sent: Tuesday, November 8, 2011 2:57:27 PM
> > Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)
> > 
> > On Tue, 8 Nov 2011, Andrei Warkentin wrote:
> > 
> > > 1 is a power of two, therefore rounddown_pow_of_two(1) should
> > > return 1. It does
> > > in case the argument is a variable but in case it's a constant it
> > > behaves
> > > wrong and returns 0. Probably nobody ever did it so this was never
> > > noticed,
> > > however net/drivers/vmxnet3 with latest GCC does and breaks on
> > > unicpu systems.
> > > 
> > > Signed-off-by: Andrei Warkentin <andreiw@...are.com>
> > > ---
> > >  include/linux/log2.h |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/include/linux/log2.h b/include/linux/log2.h
> > > index 25b8086..ccda848 100644
> > > --- a/include/linux/log2.h
> > > +++ b/include/linux/log2.h
> > > @@ -185,7 +185,7 @@ unsigned long __rounddown_pow_of_two(unsigned
> > > long n)
> > >  #define rounddown_pow_of_two(n)			\
> > >  (						\
> > >  	__builtin_constant_p(n) ? (		\
> > > -		(n == 1) ? 0 :			\
> > > +		(n == 1) ? 1 :			\
> > >  		(1UL << ilog2(n))) :		\
> > >  	__rounddown_pow_of_two(n)		\
> > >   )
> > > 
> > 
> > For what it's worth; looks good to me.
> 
> Is that an Acked-by or a Reviewed-by?
> 
I don't really know (to be honest). I guess it is whichever is weakest, 
since I did review the patch and convinced myself that the change was 
sane, but didn't spent a whole lot of time checking that *all* users were 
OK with it...


-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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